Review of Oxidative Processes in Wine and Value of Reduction Potentials in Enology

Abstract

The reduction potential of wines has been thought to indicate their level of oxidation or reduction, but how that relates to wine composition has remained vague. Reduction potentials are generated by redox couples, which are at equilibrium and both adsorbed on the measuring electrode, the magnitude of the potential for any couple being determined by the relative proportion of the oxidized and reduced component. However, redox couples associated with polyphenols, which are most likely to determine reduction potentials, are not at equilibrium in wine due to the instability of quinones. Reduction potentials are highly dependent on oxygen concentration and it is proposed that they are generated by the oxidation of ethanol coupled to the reduction of protons or of oxygen. While the so-called reduction potential is therefore of little value for wine, cyclic voltammetry has proved very useful in determining the reduction potential of wine constituents and estimating the concentration of the most readily reduced polyphenols. The reduction potentials of proposed redox couples involved in the reduction of oxygen and oxidation of polyphenols, ethanol, and sulfite are useful in determining the thermodynamic feasibility of possible interactions. The reaction of polyphenols with oxygen is mediated by iron, which redox cycles between them assisted by copper. Catechol-quinone and oxygen-hydrogen peroxide couples have similar reduction potentials. Consequently, the oxidation of catechols such as (+)-catechin cannot proceed to completion and is accelerated by substances that react with quinones, such as sulfite, which reduces them back to polyphenols. Sulfite, therefore, has multiple antioxidant activities. It accelerates oxygen consumption, prevents loss of polyphenols by regenerating them, and intercepts hydrogen peroxide, thus preventing ethanol oxidation.